1. Field of the Invention
The invention generally relates to a leak testing process. In one aspect, the invention relates to a multi-part leak testing process that employs helium while in another aspect, the invention relates to a recovery and recycling apparatus for use in the process.
2. Description of the Related Art
Leak testing processes that employ a tracer gas are often utilized to detect cracks, apertures, porousness, and the like in test parts. Such leak testing processes typically involve pressurizing the test part with the tracer gas, applying either a vacuum or a “sniff test” method, and then monitoring the test part for a leak or leaks. Monitoring of the test part can include attempting to detect any of the tracer gas that may be escaping from the test part and/or by measuring a flow rate of the tracer gas entering the test part via a leak or leaks. Commonly, prior to pressurization, the test parts are evacuated or purged with tracer gas such that they are free of any air or other gases that may have been trapped within the test part.
Tracer gases that are commonly found in leak testing processes include pure helium (i.e., virgin helium) and helium-containing mixtures having a helium concentration of about 90% to about 99.995% helium by volume. These helium-containing mixtures can include, for example, a helium-nitrogen mixture, a helium-argon mixture, a helium-air mixture, or a helium-carbon dioxide mixture. Helium is the preferred tracer gas since it is capable of quickly infiltrating the smallest cracks, crevices, apertures, and the like in the test parts. Helium is also capable of quick and easy detection by an analyzer (e.g., a helium detector) and is completely inert and non-reactive.
Undesirably, in conventional leak testing processes, helium is usually vented to the atmosphere after having been used only once. Since helium is a non-renewable resource and is produced as a by-product of natural gas production, helium can be quite expensive. To improve the cost effectiveness of using helium, various systems for recovering and recycling the gas have been attempted. For example, in U.S. Pat. No. 6,119,507 to Flosbach, et. al. a method and apparatus is described that recovers a test gas from a single test chamber (i.e., a single test part) by creating a pressure differential between the test chamber and the low pressure storage, and by utilizing a vacuum pump is described. The recovered gas is then re-compressed and stored in a high-pressure storage device. The purity of the recovered test gas is measured and, if the helium concentration is too low, an amount of the recovered (i.e., contaminated) test gas is vented from the system. The vented test gas is thereafter replaced with an equal amount of fresh test gas. Alternatively, a compensation in the sensitivity of leak testing equipment can be made based on the purity of the test gas. As a further example, in U.S. Pat. No. 5,390,533 to Schulte, et. al., a process and system for pressurizing a vessel with a helium-containing gas, recovering the gas, and then purifying the gas for reuse is described.
Unfortunately, known leak testing processes that include recovery and recycling systems have produced results that lack accuracy or stable purity control of the recovered and recycled helium. As such, the helium concentration (i.e., purity) of the recovered and recycled helium progressively deteriorates with time due to a build-up of contaminants (e.g., impurities, debris, other gases, etc.). All too often, depressurization of the recovery and recycling system is therefore required to permit the contaminated gas to purge from the system and to be replaced with fresh helium (e.g., pure helium, virgin helium, or helium with an acceptable helium concentration).
In addition, the typical leak testing processes often involve multiple, randomly-operated test parts that have different internal volumes existing at dissimilar test pressures. Therefore, when a conventional leak testing process having a recovery and recycling system is operated, the recycled helium delivery pressure is highly variable due to fluctuating flow demand. Even relatively minor changes in tracer gas composition, purity, and/or delivery pressure have a negative impact on production efficiency and leak testing process stability. This is unacceptable in a high speed, high-volume production environment, and it often results in the uneconomical use of helium.
Also, leak testing processes that include recovery and recycling systems often have a discontinuous process behavior. In other words, a flow of the tracer gas into the process is not the same as a flow of the tracer gas out of the process. As a result, during recycling of the tracer gas, problems can arise. For example, the level of impurities introduced into the leak testing process increases as a flow of the recycled gas increases and decreases as the flow of the recycled gas decreases. Moreover, a pressure in a buffer tank at the outlet of the recovery and recycling system can fluctuate as a result of the buffer tank being either over-filled or under-filled as a randomly fluctuating flow of recovered gas is introduced into the system.
Thus, a multiple test part leak testing process having a tracer gas recovery and recycling system that can increase a tracer gas recovery rate and permit tracer gas purity and delivery pressure to be maintained, regardless of tracer gas flow demand, is desirable. Likewise, a multiple test part leak testing process having a tracer gas recovery and recycling system that can overcome the effect of tracer gas purity fluctuation is desirable.